Why did notes on my iPhone's Notes App disappear?

Maybe some screen captures to better illustrate

could it be that you somehow signed out of Notes or the toggle for the Notes cloud syncing is off in which case if you know, Ted been in the cloud, maybe they are still there but not on your device visibly because when you talk off cloud syncing the disappear except for the ones on device. Probably not the problem but more information would be helpful.

You could also log into your iCloud account on the web from a PC or Mac or using your phone or tablet and check Weather notes are shown in the cloud account off of your device

You might also check your total cloud storage amount compared to your 5 GB plan. If you have exceeded, it may be certain information isn’t showing. I’ve encountered that through a software update or some other abuse or what not you went beyond the limit that you were under before storing it maybe pay the $.99 to increase your storage to allow for back up and then clean things up or restore back up if you had already created one. But a quick assessment of whether you do or don’t have any space and determination whether a dollar is worth the effort to see what might exist beyond your device and what you can see in the cloud versus your device.

Just recently a family member of mine was not able to access information on their phone and I discovered their was exceeded but fortunately due to a credit card change in their profile quite some time ago. The information is still available at Apple and upon paying the extra money it can be accessed through other than the cloud onto the device. Probably no guarantee that apples keeping anything beyond the storage you have but we’ve been fortunate and two occasions have regained information that was otherwise lost due to credit card payment method change without the family member realizing they needed to update the Apple information. That may provide some additional information to help you.

You didn’t really say whether what you had were in the cloud or on the device that you could create a new note and that you had some restoration of folders and deleted items, but not where they were stored to begin with or where your seeking to find them